Fix b/16619976: Adaptor feeds improper DFS share ACLs

DFS configuration allow the administrator to set explicit
permissions on DFS links.  The adaptor now checks for explicit
permissions returned from DfsGetInfo level 150.  If explicit
permissions were provided, they will be fed as the DFS share ACL.
If not specified, we fall back to the permissions on the namespace
root as before.

Code Review:  http://codereview.appspot.com/169840043